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The Halogens (Carius).—The method of Can"us, which is
usually employed, consists in oxidising the substance with fuming
nitric acid under pressure in presence of silver nitrate. The
silver halide which is formed is then separated by filtration and

The following apparatus is required : —

i. A piece of thick-walled soft tubing about 45—48 cm.

iS__19 in.) long, and 12—13 mm. inside diameter, the walls

being at least 2-5—3 mm. thick. Tubes of hard potash glass are
also used, in which case the thickness of the walls may be rather
less. The tube is carefully sealed at one end so that there
is no thickening of the glass at any point into a blob. If a
blob is formed, it may be removed by heating it and blowing

FIG. 19.

gently into the tube and repeating the operation if necessary.
Tubes of soft or hard glass may be bought ready sealed at-
one end. The tube is. washed out and dried before use.
2.  A narrow weighing-tube, 8—ro cm. (3—4 in.) long and
sealed at one end, which will slip easily into the thick-walled
3.  Pure fuming nitric acid of sp. gr. 1-5.—This is prepared
by distilling equal volumes of concentrated nitric acid (150 c.c.),
and concentrated sulphuric acid (150 c.c.) from a litre retort, the
neek of which has been bent in the blow-pipe flame as in Fig. 19.
The object of this bend is to prevent acid from spirting into the
neck and being carried over mechanically into  the  receiver
during distillation.     The retort is placed on a sand-bath, and